.\"	$OpenBSD: SSL_CTX_set_session_id_context.3,v 1.5 2018/03/21 17:58:58 schwarze Exp $
.\"	OpenSSL b97fdb57 Nov 11 09:33:09 2016 +0100
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.Dd $Mdocdate: March 21 2018 $
.Dt SSL_CTX_SET_SESSION_ID_CONTEXT 3
.Os
.Sh NAME
.Nm SSL_CTX_set_session_id_context ,
.Nm SSL_set_session_id_context
.Nd set context within which session can be reused (server side only)
.Sh SYNOPSIS
.In openssl/ssl.h
.Ft int
.Fo SSL_CTX_set_session_id_context
.Fa "SSL_CTX *ctx"
.Fa "const unsigned char *sid_ctx"
.Fa "unsigned int sid_ctx_len"
.Fc
.Ft int
.Fo SSL_set_session_id_context
.Fa "SSL *ssl"
.Fa "const unsigned char *sid_ctx"
.Fa "unsigned int sid_ctx_len"
.Fc
.Sh DESCRIPTION
.Fn SSL_CTX_set_session_id_context
sets the context
.Fa sid_ctx
of length
.Fa sid_ctx_len
within which a session can be reused for the
.Fa ctx
object.
.Pp
.Fn SSL_set_session_id_context
sets the context
.Fa sid_ctx
of length
.Fa sid_ctx_len
within which a session can be reused for the
.Fa ssl
object.
.Sh NOTES
Sessions are generated within a certain context.
When exporting/importing sessions with
.Xr i2d_SSL_SESSION 3
and
.Xr d2i_SSL_SESSION 3 ,
it would be possible to re-import a session generated from another context
(e.g., another application), which might lead to malfunctions.
Therefore each application must set its own session id context
.Fa sid_ctx
which is used to distinguish the contexts and is stored in exported sessions.
The
.Fa sid_ctx
can be any kind of binary data with a given length; it is therefore possible
to use, for instance, the name of the application, the hostname, the service
name...
.Pp
The session id context becomes part of the session.
The session id context is set by the SSL/TLS server.
The
.Fn SSL_CTX_set_session_id_context
and
.Fn SSL_set_session_id_context
functions are therefore only useful on the server side.
.Pp
OpenSSL clients will check the session id context returned by the server when
reusing a session.
.Pp
The maximum length of the
.Fa sid_ctx
is limited to
.Dv SSL_MAX_SSL_SESSION_ID_LENGTH .
.Sh WARNINGS
If the session id context is not set on an SSL/TLS server and client
certificates are used, stored sessions will not be reused but a fatal error
will be flagged and the handshake will fail.
.Pp
If a server returns a different session id context to an OpenSSL client
when reusing a session, an error will be flagged and the handshake will
fail.
OpenSSL servers will always return the correct session id context,
as an OpenSSL server checks the session id context itself before reusing
a session as described above.
.Sh RETURN VALUES
.Fn SSL_CTX_set_session_id_context
and
.Fn SSL_set_session_id_context
return the following values:
.Bl -tag -width Ds
.It 0
The length
.Fa sid_ctx_len
of the session id context
.Fa sid_ctx
exceeded
the maximum allowed length of
.Dv SSL_MAX_SSL_SESSION_ID_LENGTH .
The error is logged to the error stack.
.It 1
The operation succeeded.
.El
.Sh SEE ALSO
.Xr ssl 3 ,
.Xr SSL_SESSION_set1_id_context 3
.Sh HISTORY
.Fn SSL_set_session_id_context
first appeared in OpenSSL 0.9.2b.
.Fn SSL_CTX_set_session_id_context
first appeared in OpenSSL 0.9.3.
Both functions have been available since
.Ox 2.6 .
